As the understanding of the mechanics of bone stabilization and the corresponding constructs have become increasingly sophisticated, systems have been developed to allow for various types of fixation of orthopedic implants relative to the bone or bone fragments with which it is used. Initially, the fastener (typically a bone screw) simply rode in a screw hole and held a corresponding plate in compression to the bone fragments by virtue of the attachment between the fastener and the bone and the relationship between the fastener head and the fastener hole within the plate. However, as the field of internal fixation has developed, different approaches to fixation have also developed to accommodate locking fixation between the plate and fastener (in which the angle of fixation is designed to capture bone fragments from common break patterns and where the fastener is locked at that angle in the plate), variable angle fixation between the plate and fastener (in which the fastener could be fixed relative to the bone at a angle determined during surgery and where the fastener was able to move relative to the plate), and finally variable angle locking fixation (which includes a fastener assembly for which the fastener could be fixed relative to the bone at a variable angle, but could also be locked relative to the implant at that angle). This particular type of fixation allows for the biological and mechanical advantages of the previous two types of fixation, but is necessarily, the most complicated to achieve, in particular where it is of great advantage to have a fixation system that is easy to implant and which is designed to avoid mal-functions which could greatly complicate the surgery during which they are implanted.
The present invention provides a variable locking fixation system specifically including an orthopedic plate that has threaded fastener holes, which can receive a locking fixed angle screw having a head including threads that can mate with the threads of the locking holes. The fixation system further includes a variable angle locking assembly with a threaded locking insert that can be driven into the threaded locking holes of the plate. The locking insert is comprised of a material that is relatively deformable compared to the threads on the head of a variable locking fastener used in the variable locking assembly. The locking insert also includes an annular flange that surrounds the torque driving recess of the locking insert, and which resides on the top surface of the plate surrounding the fastener hole. This flange acts to help inhibit the possibility of the locking insert being driven through the plate during implantation. The fastener driver, in this case, a screwdriver, of the present invention further helps to inhibit this possibility, as well as the possibility of driving the screw too far within the locking insert, by providing a drive mechanism that will disengage at a determined depth. In particular, the screwdriver has a sleeve linked to a drive couple that disengages axially when the sleeve terminus encounters a surface, i.e. the top of the locking insert.
The screwdriver of the present invention further includes a replacement drive shaft and tip unit, which can be slide into position in the screwdriver drive body. This drive tip unit includes a variable angle drive tip that allows the screw to be inserted at the desired angle while permitting the body of the screwdriver to be used at a variable angle relative to the axis of the screw hole into which the screw is being inserted. This is of particular advantage where the screw may be difficult to access, and the room for maneuvering is small, such as is encountered in orthopedic surgery which involves the small bones, (i.e. those bones distal to the knee or elbow.)